1. Field of the Invention
The present invention relates to deterministic type packet-switching transmission networks.
A packet-switching transmission network enables the exchange of data in the form of packets between different geographically dispersed entities. Its value lies in the fact that it reduces the number of physical transmission links needed to convey information by enabling the time-sharing of one or more physical links by several information flows on certain portions of their paths.
A packet-switching transmission network consists of a set of interconnection nodes joined by transmission links that may or may not be wired. These nodes constitute a meshing of the space in which the entities that have to communicate are distributed.
A packet takes the form of a bit stream whose constitution complies with a strict organization, defined by a network, having different parts or fields. Some of these parts or fields are reserved for service information needed to convey the packet, for example the identities of the sender entity and the addressee entity. Other parts or fields are reserved for the data to be transmitted.
A packet is introduced into the transmission network at one of its interconnection nodes directly linked with the sender entity or by means of a physical transmission link such as a cable or other type of link. It travels up to the first interconnection node through the physical link connecting this first node to the sender entity. Once it reaches this first interconnection node, it is rerouted to another physical transmission link. This other physical transmission link makes this packet move forward gradually within the transmission network toward the addressee entity and enables it to reach either the addressee entity or another connection node of the network closer to the addressee identity. This other node, in turn, reroutes it to another physical transmission link, and so on and so forth. In actual fact, the packet, in its journey up to the addressee entity, follows a path that is called a virtual path because it does not take concrete form except for the time during which the packet is being transmitted. This virtual path follows a variably lengthy chain of physical transmission links joined at their ends by interconnection nodes. Each interconnection node, at its level, routes the packets that reach it between the different physical transmission links that are directly connected to it. This routing is done by means of the service information contained in the packets. A very widespread example of packet-switching networks is that of switched Ethernet networks.
In a packet-switching transmission network, the activity of the interconnection nodes is highly variable and depends on the routing of the packets. Thus, at certain points in time, there may be interconnection nodes that are close to saturation or even saturated, prompting the loss of packets, while the other interconnection nodes will be under-exploited. This has led to the real-time monitoring of the activities of the different connection nodes and to the adoption of various procedures for the local rerouting of the packets so as to better distribute the tasks between the different interconnection nodes. The price paid for this local rerouting is that the virtual path followed by a packet from its sender entity to its addressee entity is no longer fully defined in advance. This makes transmission less reliable. Above all, it adds a random factor to the time taken for a piece of information to travel through the network. In a certain number of situations, where the reliability of the transmission and the information transit time are critically important data, as in the case of the transmission network connecting the different items of equipment of an aircraft, this local rerouting is avoided, and each connection node contains a table that strictly defines the output port to be taken by a packet as a function of its input port and of the sender address and the addressee address. The packet-switching transmission network is then called a “deterministic” network because the virtual paths that may be taken by the packets are fixed and, in order to be modified, require reprogramming of the interconnection nodes and because the time taken to cross each interconnection node is limited.
2. Description of the Prior Art
However, it is not enough for the packet-switching transmission network to be deterministic in order to ensure its reliability. This network should also be sized in such a way that it is adapted to the flow of information to be transmitted, i.e. in such a way that there is no possibility of its being congested at its interconnection nodes.
An interconnection node may be symbolized by a device having:                a bank of input ports Ei, with I ranging from 1 to n, a bank of output ports, Sj, with j ranging from 1 to n,        a bank of multiplexers Pj, one per output port Sj, each multiplexer Pj being assigned to a determined output port Sj and connecting, to its assigned output port Sj, all the input port's Ei that could be connected to it,        a bank of FIFO (First In First Out) type memories Fj, interposed between the outputs of the multiplexers and the output port Sj to manage the queues directly leading to the output ports and to regularize the bit rates of the packets on the physical transmission links connected to the output port Sj, and        one or more routing automations providing for the control of the multiplexer or different multiplexers as a function of the service information contained in the packets.        
This representation of an interconnection node is designed solely for easier understanding. It does not prejudge the real architecture in which there may be only one central multiplexer that routes the flows arriving from the input ports to the appropriate output ports.
The problem of the congestion of an interconnection node brings us to that of the management of the queues, namely the occupancy rates and the risks of overflow of the FIFO memories positioned directly on output ports of the interconnection node. The transmission network must be sized so that the FIFO memories of its different interconnection nodes cannot overflow and so that they have uniform capacities and filling rates, the time taken to route a packet to an interconnection node consisting essentially of its time of stay in the queue of the output port that it takes.
The sizing of a deterministic type of packet-switching transmission network is done by a process of rough trimming and revision. The operation starts from a network topology assumed to be adapted to the geographical position of the pieces of equipment to be connected and to the size of the information flows to be exchanged. This network topology consists of the definition of virtual paths VC for conveying the different information flows, and of the meshing of interconnection nodes connected to one another and to the items of equipment by physical connection links that carry these virtual paths. It is ascertained then that the number, capacities and arrangements of the interconnection nodes and of the physical transmission links connecting the interconnection nodes to one another and to the sender and addressee entities enable problem-free passage along all the planned virtual paths. The topology of the network is revised so long as this verification does not give satisfactory results.
The packets of an information flow: coming from one and the same sender entity and occupying one and the same virtual path EC originally occupy periodic time windows that are highly spaced out with respect to the transmission capacities of the physical links used by a network. However, as soon as they pass through a first interconnection node, they enter into competition with packets belonging to other information flows following other virtual paths and may therefore be forced to wait in queues at the output port that they have to take. Such a passage through a queue disturbs the regularity of the initial bit rate of the packets. This disturbance or jitter increases with the connection nodes crossed and may ultimately give rise to packet aggregates and bursts along the virtual paths. These packet aggregates, when they go through a connection node, cause a temporary increase in the activity of this connection node. This temporary increase in activity is absorbed by the queues and gives rise to fresh delays and a possible increase in the aggregates. This phenomenon of aggregates must be taken into account when counting the virtual paths and determining the capacities of the FIFO memories of the interconnection nodes for it affects the maximum transmission time for a virtual path and the filling of the queues in the interconnection nodes.